Spatial light modulators can modulate the intensity or phase of input light in each of a plurality of two-dimensionally arrayed pixels. Such spatial light modulators include an intensity modulation type spatial light modulator that can modulate only the intensity, a phase modulation type spatial light modulator that can modulate only the phase, and an intensity and phase modulation type spatial light modulator that can modulate both of the intensity and phase. Light output after being modulated in intensity or phase in each pixel of the spatial light modulator, as a result of, for example, being condensed by a condensing optical system provided at a subsequent stage of the spatial light modulator, can process an object existing at its condensing position.
The intensity modulating spatial light modulator adjusts the transmittance of input light pixel by pixel, and cannot use light of a part that has not been transmitted therethrough, and is thus inferior in light utilization efficiency. It is not easy for the intensity and phase modulating spatial light modulator to control intensity modulation and phase modulation in each pixel independently of each other, and handling thereof is difficult.
On the other hand, the phase modulating spatial light modulator adjusts a phase change of input light pixel by pixel, and can output almost entire light, and is thus excellent in light utilization efficiency. Moreover, the phase modulating spatial light modulator, as a result of presenting a phase pattern prepared from a computer-generated hologram or the like, has a high degree of freedom in the phase distribution in a beam section of output light, and has a high degree of freedom in the condensing position of output light by the condensing optical system. As an application of light control using such a phase modulating spatial light modulator, processing of the surface and interior of a processing object, generation of a Laguerre-Gaussian mode beam, and the like can be mentioned.
Moreover, it has been known that the intensity of outputting light that is phase-modulated pixel by pixel in the phase modulating spatial light modulator can be modulated (refer to Non-Patent Literature 1). This is for causing the phase modulating spatial light modulator to present a phase pattern produced by superimposing a blazed grating pattern for light diffraction and a phase pattern having a predetermined phase modulation distribution, and adjusting the light diffraction efficiency in the spatial light modulator by adjusting the blazed grating pattern. Accordingly, it has been considered that light that is output after being diffracted by the spatial light modulator can have a desired intensity distribution and phase distribution in its beam section.
Moreover, it has been considered that, generally, since the phase α of a light wave is equivalent to a phase (α+2nπ), it is sufficient that optical phase modulation in each pixel of the spatial light modulator is possible in a range of 2π. Here, n is an arbitrary integer. For example, when the phase modulation amount exceeds 2π, it suffices to add or subtract 2nπ with respect to the phase modulation amount (hereinafter, referred to as “phase folding”) to thereby make the phase modulation amount a value within a range from 0 to 2π. It has been considered that, even if the phase modulation amount after phase folding is thus provided as the phase modulation amount of each pixel of the spatial light modulator, no problem arises in principle.
Conventional spatial light modulators are set so as to have a phase modulation range of 2π. This is because, if the phase modulation range in the spatial light modulator is 2π, a phase modulation exceeding 2π can also be expressed in principle by performing phase folding in the phase pattern. Moreover, this is because a spatial light modulator having a phase modulation range exceeding 2π is not only redundant, but also causes a reduction in resolution and a reduction in response speed in terms of the relationship between the input gradation value and phase modulation amount.